Feature, Subtract fee from amount + RBF, Important for privacy. Offering 1.5m sat. ($350)

[Transisto]

Somewhere on this tab could be a "Subtract fees from amount" checkbox.

[ecdsa]

@Transisto what do you mean?

[Transisto]

BTCPAY

Bitcoin Core

It would be extremely useful for when a customer is asking to increase his transaction priority via RBF and has already paid with cash and left.

Having the option in RBF panel would also increase privacy chain wide by removing the general assumption that the extra fee for the new RBF transaction is taken from the change.

I'm offering a tip of 100 000 sat for that feature as an option in the RBF panel.

[Transisto]

Sorry I meant 1m sat, bumping it to 1.5m sat (~350USD at of now)

[SomberNight]

To be clear, do you mean there could be an option for this in the "bump fee dialog"? So only when increasing the fee of an existing tx? -- as opposed to during initial construction of a tx

[Transisto]

I mean it'd be great if fee could be taken from amount when creating the first transaction, like the wallets above, but for us that can already be calculated easily, What's more useful for us is to make the customer pay after he left but call to request a speed up.

But yeah, the option doesn't need to be part of the fee increase dialog if the wallet just remember where to take the fee when it was first created.

BTW Bitcoin core take into account pay to many and will take the fee evenly from all outputs.

[SomberNight]

BTW Bitcoin core take into account pay to many and will take the fee evenly from all outputs.

What does "evenly" mean?
Is the amount taken away linearly scaled according to the output values,
or are all outputs decreased by the same value?

[Transisto]

Good point, it should be based on data weight of the utxo but I think disregarding the address type is good enough, although I think we should consider doing the same thing as Bitcoin Core to avoid wallet fingerprints. `subtractfeefrom' https://chainquery.com/bitcoin-cli/sendmany

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[SomberNight]

I think adding this "subtract fee from payment amount" option to the Send tab for normal tx creation is overkill. The Send tab is already versatile enough that you can achieve the same effect manually.

However, you can not achieve this currently with bump_fee. I think we could add such an option to the bump_fee dialog.
We could add a checkbox to either the dialog itself, or to Preferences, e.g. "decrease payment amounts".
If there is only one non-ismine output, it is clear what it should do.

So then the question is, what to do if there are multiple non-ismine outputs -- so for Pay to many / batched payments.
I am afraid that whatever answer we come up with, there will be users who complain that they want something else. So at least we should come up with an answer that we expect would satisfy most users.
(Maybe in the future we could have some complex expert screen that allows manually editing all amounts, but until then...)

[Transisto]

I'm not sure what non-isimine means but if it means, of different amount then I'd say only data weight of the output matters.

Transactions should be able to pay for their own weight and when batching someone buying for 50$ pays the same network fee as someone buying for 5000$

[SomberNight]

the question is, what to do if there are multiple non-ismine outputs -- so for Pay to many / batched payments.

I'm not sure what non-isimine means but if it means, of different amount then I'd say only data weight of the output matters.

An address is ismine if it belongs to the wallet. A tx input or a tx output is ismine if the corresponding address is ismine.

So what I meant there is: the question is what to do if there are multiple external/foreign outputs.

[SomberNight]

I think a variant of this is now implemented with #7026.
That PR also made it easier to add more "strategies" later, if we can come up with ones that are useful.

Feel free to test and say if it does what you wanted.
If you like it, I can share an address or LN invoice privately for your bounty :P -- it's up to you though; I think it was a useful change either way.